(1) Field of the Invention
The present invention relates generally to a method and apparatus for measuring and monitoring physiological events in humans and animals, and more particularly to a non-contact, non-invasive method and apparatus for continuously measuring and monitoring physiological events in humans or animals using a laser Doppler vibrometer to create waveforms which are directly related to the physiological events.
(2) Description of the Prior Art
The measurement of physiological events, such as blood pressure, heart rate, temperature and respiratory rate, is fundamental to determining the fitness and wellbeing of humans and animals. The continuous recording and analysis of an accurate blood pressure waveform identifies important events in the cardiac cycle, such as a subject's heart rate, the timing of peak systole, the dicrotic notch, the pre-ejection period (PEP) and the left ventricular ejection time (LVET). The location of the dicrotic notch indicates the closure of the aortic valve, which occurs at the end of left ventricular ejection, representing the end of the systolic phase and the start of diastole and left ventricular relaxation. Information about the systolic time intervals is useful in assessing cardiac condition and various disease states, including left ventricular failure, myocardial infarction, coronary artery disease, and valve disorders.
The time intervals of the various stages of the cardiac cycle are also measured to detect changes under cardiac disease conditions and pharmacological influence. For example, continuous monitoring of pre-ejection period and left ventricular ejection time ratios may be utilized to test the effects of drugs, exercise, or other stimuli, whereby an increase or decrease in the ratio may indicate an improvement or worsening of systolic efficiency.
The three basic systolic time intervals are the pre-ejection period (PEP), left ventricular ejection time (LVET) and total electromechanical systole (QS2). Linear relationships between heart rate (HR) and the duration of the systolic phases of the left ventricle (LV) have been derived by observation. These following equations have been utilized in the prior art to predict the durations of the systolic time intervals for normal patient observations based on the heart rate alone:PEP=−0.0004*HR+0.126   (1)LVET=−0.0016*HR+0.394   (2)QS2=−0.020*HR+0.522   (3)
The dicrotic notch as observed on a blood pressure waveform indicates the occurrence of the closure of the aortic valve and marks the end of left ventricular ejection. This event represents the end of the systolic phase and the start of diastole and left ventricular relaxation. The location of the dicrotic notch on a blood pressure waveform can be used for evaluating the above listed linear regression equations that may be utilized to predict the systolic time interval as a function of heart rate. The regression equations are expected to deviate for patients with cardiac dysfunction.
Traditionally, the measurement of arterial or blood pressure waveforms in humans or animals is done either by non-invasive methods that make physical contact with the patient or by invasive methods that require penetration of a patient's dermis. For example, a noninvasive method of measuring blood pressure in small animals uses a sphygmomanometer cuff wrapped around the subject's foreleg, foot or the base of the tail. As the cuff is being inflated, an ultrasonic probe is used to hear the sounds that correspond to the systolic end-points that are used to determine the corresponding blood pressure value. This method provides only systolic pressure values for a moment in time, and does not provide a time-continuous pressure waveform. There is not yet a method to measure diastolic pressure in small animals.
Alternatively, an invasive method of measuring blood pressure utilizes a monitoring system having intra-arterial catheters containing miniature pressure transducers for continuous monitoring of arterial pressure waveforms. These devices are also capable of measuring actual pressure amplitudes in time, but by their design are inserted into the arterial system, which may cause distress.
Typically, monitoring of the blood pressure waveform for animals is not routinely done, even though high blood pressure in animals can be symptomatic of a variety of diseases including chronic renal failure, hyperthyroidism, Cushing's disease, diabetes mellitus, acromegaly, Glomerular disease, polycythemia, and pheochromocytoma.
There exists a need to continuously and accurately measure blood pressure without making physical contact with the subject, especially for patients, such as burn victims, neonates, and patients who need to be monitored without disturbing sleep or rest. There is also a need to take such measurements on a subject that may be prone to sporadic movement. The proposed laser-based, non-contact and noninvasive technique is capable of measuring the arterial pressure waveform from which the timing of various events in the cardiac cycle can be determined, without causing additional distress or discomfort.